This invention relates to engines.
This invention has particular application to methods of and apparatus for converting standard four-stroke engines into efficient two-stroke engines. However this invention is not limited to converting engines and may be applied to the original production of an efficient two-stroke engine.
There are prior disclosures of two-stroke engines which utilise power cylinders charged from a pumping chamber to provide increases in efficiency. However inherent in such proposals is the high cost of re-tooling for an all new engine design. Furthermore it is considered that many of these earlier proposals may not meet the stringent emission standards now required of most internal combustion engines. For example, it is very desirable to reduce emissions of oxides of nitrogen (NOx) and particulates including soot. Efficiency in terms of such emission reductions can be more important than fuel efficiency or achieving power gains.
The existing engine industry is large, mature, stable and conservative. The barriers to entry for even modest changes to engine design are formidable. Engine buyers are committed to existing engines and engine design. They are tooled up with expensive plant and equipment for conventional engines and are more likely to accept technological advances of an incremental nature, as opposed to radical departures.
This invention in one aspect aims to provide methods of and apparatus for converting standard four stroke engines into two-stroke engines which may operate efficiently in terms of selected or all exhaust emissions, fuel efficiency and power output from the converted engine. This invention also aims to provide engines which are useful and which have commercial appeal to both manufacturers and users.
With the foregoing in view this invention in one aspect resides broadly in a method of converting a four-stroke reciprocating piston engine into a Two-stroke engine including:
providing a reciprocating positive displacement pump having a respective pumping chamber for groups of at least two cylinders of the engine, each pumping chamber having a displacement swept by its pumping piston which is greater than the swept cylinder displacement of each cylinder of the engine;
securing the pump to a mounting on the engine adjacent the cylinders whereby the outlet from the pump is located closely adjacent the inlets of the engine;
arranging the crank pins for each group of cylinders at angular spacings of 360xc2x0 divided by the number of cylinders in the group;
providing step-up drive means for driving the pump from the engine, the step-up being in the ratio of the number of cylinders in each group of cylinders of the engine per pumping chamber;
providing relatively short feed passages through transfer manifolding interconnecting the outlet from each pumping chamber to the inlets of the group of cylinders to be fed thereby, and
timing the connection between the engine and the pump and the operation of the inlet and exhaust valves of the engine such that:
the or each pumping piston leads alternate ones of the power pistons fed thereby to their respective Top Dead Centre (TDC) positions;
the inlet valve to each power cylinder to be fed opens before Bottom Dead Centre (BDC) and closes before TDC, and
the outlet valve from the fed power cylinder opens before BDC and closes before TDC.
Preferably:
the or each pumping piston leads alternate ones of the fed power pistons to Top Dead Centre (TDC) position by 80xc2x0 to 160xc2x0 of crankshaft rotation;
the inlet valve to the power cylinder to be fed opens in the range 50xc2x0 to 0xc2x0 before BDC;
the inlet valve to the power cylinder to be fed closes in the range 70xc2x0 to 160xc2x0 before TDC of crankshaft rotation;
the outlet valve from the fed power cylinder opens in the range 110xc2x0 to 40xc2x0 before BDC, and
the outlet valve from the fed power cylinder closes in the range 100xc2x0 to 180xc2x0 before TDC of crankshaft rotation.
In the above ranges the timings closer to BDC would be more suitable for engines which operate at relatively low operating speeds and particularly large engines. High speed engines would advantageously operate at the other end of the range.
For a typical two litre automotive diesel engine converted or operating to this cycle and optimised to operate at a synchronous speed of 1500 RPM for driving, a 24 OV alternator for example, the typical timings would be:
the pumping piston leads the power piston to top dead centre by 120xc2x0;
the inlet valve to the power cylinder to be fed opens at 40xc2x0 before bottom dead centre and closes at 110xc2x0 before top dead centre;
the outlet valve from the fed power cylinder opens at 70xc2x0 before bottom dead centre and closes at 140xc2x0 before top dead centre.
For a typical two litre automotive diesel engine converted or operating to this cycle and optimised for high speed, typical timings would be:
the pumping piston leads the power piston to top dead centre by 135xc2x0;
the inlet valve to the power cylinder to be fed opens at 45xc2x0 before bottom dead centre and closes at 115xc2x0 before top dead centre;
the outlet valve from the fed power cylinder opens at 85xc2x0 before bottom dead centre and closes at 155xc2x0 before top dead centre.
Step-up ratios of two to one for the driveshaft relative to the crankshaft are preferred for high speed engines in order that effective transfer of air from pump to power cylinder may be achieved. Step-up ratios of more than two to one are preferably limited to relatively slow speed and medium speed engines.
Suitably the swept volume of the pumping chamber is less than 1.6 times greater than each respective power cylinder. For example in applications requiring modest power gain the pumping chamber swept volume may be up to 30% greater than the swept volume of each respective power cylinder. In applications for high power gains the swept volume of the pumping chamber may be up to 60% greater than the swept volume of each respective power cylinder.
Preferably for greater emission improvements the swept volume of the pumping chamber may be 60% greater than the swept volume of each respective power cylinder swept volume.
Furthermore the pump components are required to operate under much lower pressures and temperatures than the power components and this invention enables the components to be optimised by having the relatively robust components of the converted engine perform work with each revolution while utilising less robust components for pumping and thus providing advantages in reduction of power consumption and an associated reduction in friction loads.
Preferably the transfer manifold or pump head is provided with a discharge valve which may be driven but which is suitably a reed valve or like pressure sensitive valve which prevents back flow of gases from the transfer manifold to the pump cylinder during the scavenging-intake phase of the power cylinder. More preferably the discharge valve is located closely adjacent the outlet from the pumping chamber minimising the re-expansion volume and thus improving the volumetric efficiency of the pumping chamber.
The provision of the discharge valve may trap a charge of pressurised fresh gas downstream of the discharge valve such that at initial opening of the inlet valve and before closing of the exhaust valve a positive flow of fresh gas is injected from the inlet manifold to enhance scavenging of the exhaust gases. This provision can also be utilised to inhibit the back flow of spent gases from the power cylinder via the transfer port and transfer manifold into the pump cylinder.
The transfer manifold from the pump to the group of cylinders may include a single upstream branch connected to the pump and communicating with a plurality of downstream branches with the cylinders of the group. In such an application a single discharge valve. such as a reed valve, may be utilised in the upstream branch for simultaneous communication with all downstream branches.
However it is preferred that the discharge valve be of a type which may be controlled to communicate in a sequential manner with alternate ones of the downstream branches. This will minimise the effective volume of the passage between the pump and the respective cylinders for more efficient gas transfer. Preferably the discharge valve is a timed rotating drum valve which is disposed as close as possible to the pump piston crown at top dead centre and which provides sequential communication with the downstream branches.
Deflector means may be provided in the inlet tract or valve shrouding or the like may be provided to induce loop type scavenging of spent exhaust gases.
It is also preferred that a reed valve or other valve means be arranged in the inlet tract to the or each pumping chamber to assist in enhancing volumetric efficiency of the pumping chambers.
In order to provide the required crankshaft/driveshaft timing the group of cylinders being fed by the one pump cylinder must have their associated crank pins at angular spacings of 360xc2x0 divided by the number of cylinders in the group. Accordingly the converted engine may require crankshaft modifications to achieve this configuration. The camshaft will require new xe2x80x98timingsxe2x80x99 to suit. The camshafts will benefit from modified lift profiles to suit the shorter exhaust/inlet phase this may also require other valve train modifications, such as spring rates. Furthermore, the oil pump may be modified to accommodate a larger oil circuit to include the bolt on pump and to maintain pressure at a lower engine idle.
It is preferred that for balance purposes respective pairs of cranks, of converted engines having multiples of two cylinders, be evenly offset from one another. That is in a conventional four cylinder engine which has the cranks contained in a common plane, the front and rear pairs of cranks be offset at 90xc2x0 to one another to producing a firing in the converted engine at every 90xc2x0 of one revolution of the crankshaft.
In another aspect this invention resides broadly in a two stroke reciprocating engine having head mounted inlet and outlet valves and an external pump for charging the cylinders, wherein:
the external pump is a reciprocating positive displacement pump having a respective pumping chamber for groups of at least two cylinders of the engine, each pumping chamber having a displacement swept by its pumping piston which is greater than the swept cylinder displacement of each cylinder of the engine;
the pump is secured to a mounting on the engine adjacent the cylinders whereby the outlet from the pump is located closely adjacent the inlets of the engine;
the crank pins for each group of cylinders are arranged at angular spacings of 360xc2x0 divided by the number of cylinders in the group.
step-up drive means is provided for driving the pump from the engine, the step-up being in the ratio of the number of cylinders in each group of cylinders of the engine per pumping chamber;
relatively short feed passages are provided through transfer manifolding interconnecting the outlet from each pumping chamber to the inlets of the group of cylinders to be fed thereby, and
the connection between the engine and the pump and the operation of the inlet and exhaust valves of the engine are timed such that:
the or each pumping piston leads alternate ones of the power pistons fed thereby to their respective Top Dead Centre (TDC) positions;
the inlet valve to each power cylinder to be fed opens before Bottom Dead Centre (BDC) and closes before TDC, and
the outlet valve from the fed power cylinder opens before BDC and closes before TDC.
In an engine with four or more cylinders, to prevent the exhaust pulse or phase of one cylinder from interfering with the scavenging phase of another cylinder, separate exhaust manifolds, or a manifold of a type which prevents interference of the exhaust phase with the scavenging phase, is provided. In the case of a turbocharged engine separate turbocharger inlets are provided or a dividing scroll is provided in the turbocharger inlet. Alternatively, separate turbochargers may be utilised.